1. Field of the Invention
The invention relates to image coding apparatus and method for efficiently coding image data.
2. Related Background Art
An MPEG (Moving Picture Experts Group) system is widely known as a system for highly efficiently compressing and coding motion image data. The MPEG system is a coding system in which a discrete cosine transformation (DCT), an adaptive quantization, and a variable length coding are used as fundamental techniques and an interframe (or interfield) predictive coding with motion compensation based on a frame (or field) correlation is adaptively combined.
Specifically speaking, an I picture in which each frame is coded in a picture plane, a P picture which is interframe (or interfield) motion compensation predictive coded from the I picture that is preceding with respect to the time, and a B picture which is interframe (or interfield) motion compensation predictive coded from both of the preceding I picture (or P picture) and the subsequent I picture (or P picture) are arranged in accordance with a proper order, thereby accomplishing a high compression ratio.
A recent image coding system has a construction which can be also called a set of various coding techniques. Among the various coding methods, there are advantages and disadvantages depending on a photographing situation or an object. However, which coding method is used is proper can be discriminated for the first time by analyzing an image signal inputted to a compression processing circuit.
The MPEG system is what is called a collective method of those composite coding methods. A camera integrated type recording and reproducing apparatus using the MPEG system has already been manufactured as an article of commerce.
According to the MPEG system, since an inter-picture plane correlation is used, a compression ratio is higher than that of a compression coding system of a conventional digital VCR for industrial use, a longer time recording can be performed with a smaller recording capacity, and it is suitable for a portable camera apparatus.
However, the MPEG system is not suitable to form a predictive coding image for a period of time of the camera operation such as zoom-in (enlargement), zoom-out (reduction), or the like. That is, since it is difficult to obtain a correlation between the picture planes during the zooming operation, a large amount of unnecessary data is generated due to prediction errors. Not only a coding efficiency deteriorates but also a picture quality deteriorates.